The present invention relates to a vacuum degassing apparatus for molten glass, which removes bubbles from molten glass continuously supplied.
In order to improve the quality of formed glass products, there has been used a vacuum degassing apparatus which removes bubbles generated in molten glass before the molten glass that has been molten in a melting tank is formed by a forming apparatus, as shown in FIG. 3.
The vacuum degassing apparatus 110 shown in FIG. 3 is used in a process wherein molten glass G in a melting vessel 112 is vacuum-degassed and is continuously supplied to a subsequent treatment vessel. In the vacuum degassing apparatus 110 are provided a vacuum housing 114 which is evacuated to be depressurized therein for vacuum-degassing the molten glass, a vacuum degassing vessel 116 which is provided in the vacuum housing 114 and is depressurized together with the vacuum housing, and an uprising pipe 118 and a downfalling pipe 120 which are connected to respective end portions of the vacuum degassing vessel in a downward and vertical direction. The uprising pipe 118 has a lower end immersed in the molten glass G in an upstream pit 122 in communication with the melting vessel 112. Likewise, the downfalling pipe 120 has a lower end immersed in the molten glass G in a downstream pit 124 in communication with the subsequent treatment vessel (not shown).
The vacuum degassing vessel 116 is substantially horizontally provided in the vacuum housing 114 which is evacuated through a suction port 114c by a vacuum pump, not shown, to be depressurized therein. Since the inside of the vacuum degassing vessel 116 is depressurized, through suction ports 116a and 116b in communication with the inside of the vacuum housing 114, to a pressure of 1/20-⅓ atm together with the inside of the vacuum housing 114, the molten glass G in the upstream pit 122 before degassing is sucked and drawn up through the uprising pipe 118, and is introduced into the vacuum degassing vessel 116. Then, the molten glass is vacuum-degassed as it flows through the vacuum degassing vessel 116, and the molten glass is drawn down by the downfalling pipe 120 to be discharged into the downstream pit 124.
The vacuum housing 114 may be a casing made of metal, such as stainless steel and heat-resisting steel. The vacuum housing is evacuated from outside by e.g. a vacuum pump (not shown) and the inside is depressurized, so that the inside of the vacuum degassing vessel 116 provided therein is depressurized and maintained under a prescribed pressure, e.g. under a pressure of 1/20-⅓ atm. In the vacuum degassing vessel 116, an upper space 116s is formed above the molten glass G filled to a certain depth in the vacuum degassing vessel. The upper space 116s is a space depressurized by a vacuum pump (not shown) so that gas components from bubbles which have risen to the surface of the molten glass G and broken up, are sucked from the upper space being the depressurized space, through a suction port 114c by a vacuum pump (not shown). Therefore, the larger the area of the molten glass G in contact with the upper space 116s is, the more remarkable the vacuum degassing effect becomes.
Around the vacuum degassing vessel 116, the uprising pipe 118 and the downfalling pipe 120 in the vacuum housing 114 is provided thermal insulation material 126, such as refractory bricks, to cover these members for thermal insulation.
Further, with a conventional vacuum degassing apparatus 110 as illustrated in FIG. 3, it is conceivable to enlarge the apparatus in an attempt to increase the flow rate i.e. the throughput capacity for degassing by constituting the vacuum degassing vessel 116 by dense refractory bricks, particularly by electro-cast refractory bricks, as disclosed in JP-A-11-240725 filed by the present applicant.
However, in order to increase the flow rate of the molten glass and to perform the desired vacuum degassing treatment, it is necessary to increase the width and the total length (namely, the bottom area) of the vacuum degassing vessel 116, and the diameters of the uprising pipe 118 and the downfalling pipe 120, by taking into consideration, changes in various factors (for example, a change in the flow rate of the molten glass G to be degassed, a change in the concentration of gas components dissolved in the molten glass G due to a temperature drop of the molten glass G in the melting furnace, or a change in the pressure in the vacuum degassing vessel which is depressurized).
However, by increasing the width and the total length of the vacuum degassing vessel 116, and the diameters of paths in the uprising pipe 118 and the downfalling pipe 120, the apparatus will be large-sized, and necessary refractory bricks, etc. will inevitably increase, thus leading to a problem of an increase of the costs.
Further, when the number of bubbles contained in the molten glass G rapidly increase, there will be a problem such that non-removed bubbles will remain in the molten glass G, which will flow into the downfalling pipe, and the bubbles are likely to remain in the glass as a product. Further, due to the increase of the number of bubbles, unbroken bubbles may build up on the surface of the molten glass G, and stick to the ceiling of the vacuum degassing vessel 116. Consequently, a volatile matter present at the ceiling, which has been solidified in the form of crystals, may be included in the molten glass G. As a result, small opaque matters may remain in the glass product and form defects which are so-called “stones”. Further, even if the volatile matter is dissolved in the high temperature molten glass G, it will not be diffused uniformly in the molten glass G, and consequently, the molten glass G may have local changes in the composition. Due to the changes, the product glass obtained from the molten glass G, may have local changes in the refractive index and the see-through image of the glass may be distorted, which is so-called deterioration of leam.
Further, in order to increase the bottom area of the path in the vacuum degassing vessel 116, a method of increasing total length of the path of the vacuum degassing vessel 116 may be conceivable. However, there is a problem such that as the size of the apparatus increases, the apparatus becomes long as compared with the melting vessel 112. Consequently, it becomes necessary to change the relative position between the melting vessel 112 being the existing facility, and the downstream pit 124, whereby there will be a demerit that the existing facility can not effectively be utilized. Further, if the vacuum degassing vessel is made linearly long, the expansion of the vacuum degassing vessel 116 by a heat, increases in proportion thereto, and there will be a change in the center to center distance between the uprising pipe 118 and the downfalling pipe 120, which creates a distortion of the apparatus and thus may deteriorate the safety.
Otherwise, in order to increase the bottom area of the vacuum degassing vessel, a method of increasing the width of the path may also be conceivable. However, it is difficult to sufficiently improve the vacuum degassing performance only by increasing the width of the path. The reason will be described with reference to FIGS. 8 and 9. FIG. 8 is a schematic cross-sectional view of the vacuum degassing apparatus 110 taken along line B-B′ in FIG. 3, and illustrates the cross-sectional shape of the path for molten glass in the vacuum degassing vessel 116. As illustrated in FIG. 8, the path for molten glass in the vacuum degassing vessel 116 in the vacuum housing 114, is formed by assembling path members 116c, and the bottom portion 116d of the path for molten glass is flat.
FIG. 9 shows a flow rate distribution of molten glass in the transverse direction of the path. As shown in FIG. 9, it is evident that the flow rate of molten glass is highest at the center in the transverse direction (hereinafter referred to as the center of the path), and on the contrary, the flow rate of molten glass is lowest at the ends in the transverse direction (hereinafter referred to as the sides of the path). For this reason, there is a possibility that the molten glass flowing in the center of the path reaches the downfalling pipe without undergoing sufficient degassing, and bubbles are thereby included in the glass product. Namely, there has been a problem such that, even if the width of the path is simply increased, the molten glass still has a low flow rate and hardly flows at the sides of the path. Therefore, the increase of the width does not remarkably contribute to improvement of the vacuum degassing performance.